JP2005047625A - Composite material for shielding microwave energy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To raise the temperature of a food uniformly without directly losing microwave energy in microwave oven heating. <P>SOLUTION: A food package 10, which is for the storage and microwave heating of the food 12, has a top panel 16, side panels 18, and a bottom panel 20 forming a food reception cavity 14. An impedance matching member 22 is arranged on at least one panel for impedance matching for microwave energy entering the package. It is desirable that the impedance matching member 22 be a continuous film made of a thin flake material embedded in a dielectric binder, whose size and shape can be determined according to that of a food. The flake material has an aspect ratio of at least 1,000, and the impedance matching member 22 including such a flake material has an effective dielectric constant of at least 4,000. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to microwave cooking of food. More particularly, the present invention impedance matches microwave energy within a microwave oven so that the microwave energy is more evenly distributed within the food product without interacting with the microwave energy to generate heat. A microwave food package including means.

  The popularity of microwave ovens for cooking all or part of a meal has led to the development of numerous food packages that can cook food directly in a microwave oven within the food package in which the food is stored. The convenience of cooking food in the package or its components has caught the attention of many consumers. However, when microwave cooking a certain food, one complaint is that the center of the food cannot be heated or warmed without scorching or severely dewatering the outside. In particular, when the amount is large, it is very difficult to uniformly heat in a microwave oven using a conventional food package. Even when the outer part is fully cooked, the center is undesirably cold.

  Microwave interactive films have been produced that can generate heat on the food surface to bake food. US Pat. No. 4,641,005 (issued to Seiferth, assigned to James River Corporation, Virginia, the assignee of this application), is a microwave interaction useful for food packages that can be baked on the surface of food. Disclosed material. In particular, the interactive material includes a very thin metal film applied to a polymer material adhered to a rigid substrate. Such films actually interact with microwave energy to generate heat on the food surface. The heat supplied by such interactive materials is beneficial for the tough baking of the food surface, but the outer part of the food is cooked faster than without the interactive material, so that the interior is not heated sufficiently. Therefore, it is inconvenient for cooking thick foods having a large dielectric constant.

  In addition, additional microwave heating devices have been developed primarily for use in food packaging. U.S. Pat. No. 4,876,423 (issued to Tighe et al.) Discloses a medium for producing localized microwave radiant heating. Here, the medium is formed from a mixture of a polymeric binder, conductor and semiconductor particles that can be applied or printed onto a substrate. However, in this case as well, such media are designed to bake the surface of the food crisply or in a scale by interacting with electromagnetic microwave energy and generating heat, but heating the food center. Do not offer to strengthen.

  A number of microwave food packages or containers have also been developed that are designed to uniformly heat or adjust the reflectivity, transmittance, or absorption of microwave energy. U.S. Pat. No. 4,266,108 (Anderson et al.) Discloses a microwave heating apparatus that includes both a microwave reflecting member and a microwave absorbing member spaced a sufficient distance to provide a temperature self-limiting device. is doing. However, as provided in the above-mentioned patents, the device includes a heating element that conducts and heats the food by interacting with microwave energy to generate heat.

  In addition, U.S. Pat. No. 4,927,991 (Wendt et al.) Relates to food packaging and discloses a regenerator or heating element in combination with a grid. Here, the regenerator surface is tuned to a matched impedance for maximum microwave power absorption. In particular, the reflectance, transmittance and absorption rate of the heating element should be adjusted by changing design factors including grid hole size, heat accumulator impedance, grid shape, grid-to-heat accumulator spacing, and adjacent hole spacing. Can do. The food intended to be cooked in such a package is similar to the one described above, in particular foods that require a somewhat crisp or crusted surface, such as pizza, fish sticks or French fries. is there. Furthermore, the problem of moderately heating the center of these foods is not required in this device because they are of a relatively thin type as a whole.

  Containers with specially designed covers or lids have also been developed. They can modify the pattern of the microwave field, and the dielectric constant can be changed during microwave heating, and the heat distribution in the container can be changed as heating proceeds. U.S. Pat. No. 4,888,459 (issued to Keefer) discloses a microwave container that includes a dielectric structure to provide these properties. In particular, this patent discloses a container that includes a lid having a single or multiple metal plates or sheets disposed thereon. An electrically denser region is formed from the dispersion of metal particles in the matrix, and the dielectric constant of this electrically high portion is disclosed to be in the range of 25 to 30 for the unrestricted region. Furthermore, this region is lossy in that at least initially this region is microwave-absorbing and thus can be heated when exposed to microwave energy. Moreover, the electrically denser regions actually have a reduced dielectric constant during microwave heating. Unfortunately, the electrically denser regions disclosed in this patent and related US Pat. No. 4,866,234 at least partially interact with microwave energy. As a result, this area will generate undesired heat during food preparation such as pot pie (deep pie) or fruit pie during microwave cooking. Furthermore, without a shut-off function, the generation of heat will also pose a risk of scorching and burning for foods that require long cooking times.

  U.S. Pat. No. 4,656,325 (Keefer) discloses a microwave heating package including a cover arrangement for use with microwave reflective food containing dishes such as aluminum foil dishes. The cover allows microwave radiation into the container containing the food and is therefore compared to an optical non-reflective coating, but substantially prevents the microwave radiation reflected from the food surface and the bottom of the container from leaking. This prevents energy from being trapped or concentrated in the container. The cover disclosed in this patent is designed to provide, among other things, a baked and / or baked food surface.

  Food wraps have also been developed for surface heating of food by changing the microwave permeability. U.S. Pat. No. 4,972,058 (Benson et al.) Consists of a porous dielectric substrate and a coating comprising a dielectric matrix and flakes of microwave sensitive material, by absorption of microwave energy. A composite material for generating heat is disclosed. The aspect ratio of the flakes is at least 10. However, the flake material used in the composite material disclosed in this patent is limited to metal flakes with jagged edges.

Accordingly, there is a need for a microwave package that has a means for uniformly and uniformly raising the temperature of food, particularly food with a high dielectric constant. In particular, a high dielectric constant microwave package element that does not interact with microwave energy to generate heat and can raise the food temperature in a predetermined region that depends on the size and shape of the element is a thick food. Is needed for.
U.S. Pat. No. 4,641,005 U.S. Pat. No. 4,876,423 U.S. Pat. No. 4,266,108 U.S. Pat. No. 4,927,991 U.S. Pat. No. 4,888,459 U.S. Pat. No. 4,866,234 U.S. Pat. No. 4,656,325 U.S. Pat. No. 4,972,058

  Accordingly, the main object of the present invention is to overcome the deficiencies of the prior art as described above, in particular for food storage and microwave heating that raise the temperature of the food without directly losing microwave energy for heating. Is to provide a package.

  Another object of the present invention provides a package including means for impedance matching microwave energy entering the package to uniformly increase the temperature of the food stored in the package, including the food center. That is. The impedance matching means does not interact with the microwave energy to generate heat.

  Yet another object of the present invention is to provide a food storage and microwave heating package that includes impedance matching means provided on a portion of the package to impedance match microwave energy entering the package. . The impedance matching means includes a continuous film of thin flake material embedded in a dielectric binder that can raise the temperature of a predetermined area of the food without interacting with microwave energy to generate heat.

  The above objective is accomplished by providing a package that includes a package body that defines a food receiving cavity. In particular, the package body has a bottom panel and a top panel and side panels that join the bottom and top panels. An impedance matching element is provided on the at least one panel for impedance matching microwave energy entering the package. The impedance matching element is a continuous film of thin flake material, preferably embedded in a dielectric binder that is sized and shaped with respect to food, and impedance matching interacts with microwave energy to generate heat. Rather, the temperature of the food within a given area is increased depending on the size and spacing of the membrane.

  As a result, the central portion of a thick food such as a pot pie is completely heated without scorching or overheating the outer portion.

  Many thick foods such as large pot pie or fruit pie are cooked faster at the edges than in the middle, so microwave cooking of food is not commercially acceptable to consumers for any cooking needs . The present invention relates to a cooking means for impedance matching microwave energy and its cooking to increase the temperature of these areas, which are normally heated slowly, by effectively coupling the microwave energy to specific areas of the food. A food package including the means is provided. By mathematical analysis, it has been determined that the impedance matching means of the present invention is more evident in loads with higher dielectric constants, and the optimal spacing for impedance matching decreases with dielectric constant but is negligible. Impedance matching is achieved by utilizing a film spaced between the food and incoming microwave radiation. The presence of the impedance matching film increases the amount of microwave energy transferred directly to the food.

  For a clearer understanding of the present invention, attention is first directed to FIG. In particular, FIG. 1 shows a food package 10. The food package 10 includes a food 12 shown as a pot pie in a food receiving space 14. A number of additional foods such as fruit pie and stews can also be effectively heated by the packages made according to the present invention.

  The food package 10 has a top panel 16, a side panel 18, and a bottom panel 20 that are substantially transparent to microwave energy and form a food receiving space 14 composed of various microwave permeable materials. Preferably the food package is made from paper or paperboard, but may be made from a microwave compatible plastic material. The impedance matching member 22 is preferably disposed on the top panel 16 above the food item 12. By placing the impedance matching member 22 above the food 12 as shown in FIG. 1, the microwave energy entering the package 10 is impedance matched by the member 22 and effectively disperses the microwave energy to the center of the food 12. Let Here, the member 22 does not interact with the microwave energy to generate heat. As a result, the member 22 is not a heater in the conventional sense, but instead provides a novel means for effectively raising the temperature inside the food by impedance matching the incident microwave energy acting on the food.

  FIG. 2 clearly shows the impedance matching member 22 disposed on the inner surface of the top panel 16 above the food product 12. Preferably, the impedance matching member 22 is located 1/8 inch to 5/8 inch (about 3.18 mm to 15.88 mm) above the surface of the food product 12. The impedance matching member 22 may be printed or coated directly on the container 10 or may be applied to another substrate in advance. The substrate is paperboard, paper, polyester film, or any other microwave transparent material that can support the impedance matching member 22.

  The food package 10 can also be designed with a number of additional configurations, some of which are shown in FIGS. In particular, FIG. 3 shows the package 10 with an impedance matching member disposed on the top panel 16 outside the package. Furthermore, the impedance matching member 22 may be disposed between different materials. For example, FIGS. 4 and 5 show the impedance matching member 22 disposed between the substrate 24 and the adhesive layer 26 used to laminate the impedance matching member to the top panel 16 of the food package 10. Yes. The substrate 24 is paper, paperboard or a thin film on which the impedance matching member 22 can be printed or coated.

  6 and 7 illustrate additional embodiments in which the impedance matching member 22 is embedded or surrounded by a thin film 28 of resin or ink applied to the surface, for example, by a conventional printing process. Furthermore, the impedance matching member 22 can be sandwiched by a material 30 such as paper, paperboard, or plastic that is adhered to the surface by an adhesive layer 26 as shown in FIGS. These embodiments are only a few of the many package configurations that can utilize the impedance matching member 22.

  FIG. 10 shows yet another possible package configuration 10. Here, the impedance matching member 22 is placed on the lid of the food tray rather than on a separate container as shown in FIGS. 1 and 2-9.

  Impedance matching member 22 includes a thin flake material film embedded or retained within a dielectric binder material. Preferably, the impedance matching member 22 is shaped to be smaller than the food 12 in the diameter direction. The dielectric binder is selected from various commercially available binder materials such as silicon or acrylic binders.

  In particular, preferred dielectric binders are materials with low loss tangent, high dielectric constant, and high dielectric strength (all evaluated at 2.45 GHz). Low loss silicon binders such as Dow Corning 1-2577 and acrylic binders such as styrene / acrylic Joncryl 611 from Johnson Wax are harmful heat in the presence of microwave energy. Is utilized to provide a coating having a desired impedance matching response without producing a. On the other hand, when a high loss tangent resin, such as nitrocellulose, is used as the binder material, the resulting impedance matching coating is overheated when exposed to microwave energy, resulting in a coating substrate Various undesirable side effects occur, such as scorching and melting.

  The thin flake material of the present invention is essential for achieving advantageous results. The flakes are generally flat and planar and are made from a metallic material. It is important that the flakes have a length that allows them to lie substantially flat within the binder material. At the same time, the flakes must be of a length that allows them to be printed on the substrate by conventional printing processes such as gravure printing. In general, the desired flakes are aluminum metals with an average longest dimension in the range of about 8-75 micrometers (μm) and a smaller dimension, ie, a width in the range of 5-35 μm. Preferably, the longest dimension is in the range of 10-30 μm. Aluminum metal is preferred, but other metal materials can be applied to the present invention as well.

  Referring to FIGS. 11 and 12, the preferred flakes of the present invention are shown enlarged. As can be seen from the drawings, the flakes themselves have a substantially smooth perimeter, and it appears that there are only a few pieces of flakes present in the binder. The apparent smoothness of the flakes will depend on the degree of expansion. However, considering the flake perimeter to be smooth can be defined by comparison with flakes having a jagged perimeter. In particular, the smoothness around the flakes can be contrasted with the flakes that are jagged to the extent that the jagged flakes include a large number of intersecting straight lines to form an angle less than 180 °. The smooth perimeter of the flake provides a smaller overall parameter length than the jagged perimeter. Figures 13-15 and 16-18 show prior art metal flakes. By comparing the flakes shown in FIGS. 13-15 and 16-18 with the flakes shown in FIGS. 11 and 12, the flakes shown in FIGS. 11 and 12 have a smaller parameter length. Is clear.

  In addition to the length, the thickness of the flake material is important in order to obtain the advantageous features of the present invention. The flakes must have sufficient thickness to maintain flake dimensional integrity and sufficient mechanical strength to withstand dispersion in the binder material. On the other hand, the flake material must not be so thick that it cannot provide a tight packing between adjacent flakes. Preferably, the flakes have a thickness in the range of 100 to 500 mm. More preferably, the flakes have a thickness in the range of about 100 to 200 mm. When the flake material consists of aluminum metal, the preferred aluminum flakes are made from aluminum metal by vapor deposition and the thickness provides an optical density in the range of 1-4.

  The flake material also preferably has an aspect ratio of at least 1000. Such an aspect ratio provides an impedance matching member 22 having an effective dielectric constant of at least 4000. With such a high dielectric constant, the thin impedance matching member 22 can match the impedance of the microwave energy present in the microwave oven, while the micro-intensity is placed under the impedance matching member 22 into the food held in the package. Wave energy can be directed more effectively.

  When these flakes are suspended and printed in a dielectric binder, the flakes form an archipelago of flat conductive islands that are in close contact at many locations and form an impedance matching member 22. This concentrates the electric field in the region between the flakes and significantly increases the amount of stored electrical energy. The impedance matching member 22 thus formed is a non-conductive film having a very high dielectric constant for all purposes and purposes.

  A quantitative representation of film capability for impedance matching is represented by a single dimensionless film parameter x. Such an expression will help in understanding the advantageous results demonstrated below. In particular, for resistive and capacitive films, x is defined as:

x = σdZ _0/2 (resistive film) (1)
_{x = πifε r ε 0 Z 0} d ( capacitive film) (2)

In these equations, Z ₀ is the free space impedance of the radiation projected onto the film plane, σ is the bulk conductivity of the resistive film, d is the film thickness, i is the square root (imaginary number) of minus 1, f is the frequency, ε ₀ is the free space dielectric constant (generally equal to 8.85 × 10 ⁻¹² farads / meter), and ε _r is the complex dielectric constant of the capacitive film.

  Returning again to the mathematical representation of the impedance matching member of the present invention, when an infinitely sized film is placed in free space, the reflection coefficient R and transmission coefficient T are given by the following equations for resistive and capacitive films: It is.

R = −x / (1 + x) (3)
T = 1 / (1 + x) (4)

In a resistive film, x is a real number, T is in phase with incoming radiation, R is 180 ° out of phase, and the absolute values of R and T are 1 in total. Since the capacitive film x is a complex number, the R and T phases is the phase depends of the size and epsilon _r of x. When summed as a complex number, T is still equal to 1 + R, but the sum of the absolute values of T and R is greater than one. Since no energy is lost in a perfect dielectric, a capacitive film with the same reflection amplitude as the resistive film will transmit more radiation. In the following discussion, it should be understood that the x value of the capacitive film is a complex number.

  The portion of the incident power that is lost in the resistive film is expressed by equation (5), and the power that is lost in the capacitive film is expressed by equation (6).

A _r = 2x / (1 + x) ² (5)
A _c = 2 | x | sin δ / (1+ | x | ² +2 | x | sin δ) (6)

  In the above formula, δ is the loss angle of the dielectric. It should be noted that the resistive film has a peak absorption of 0.5 at x = 1 and the capacitive film has a peak absorption of sinδ / (1 + sinδ) at | x | = 1. A perfect dielectric (sinδ = 0) has no absorption for any size of x. It should also be noted that these equations are only applicable to thin films, which means that the film thickness must be much smaller than the wavelength of radiation in the film. .

The power distribution in thin film radiation is calculated using a simple electrical network. The incident radiation is expressed as a source with a free space output impedance (Z ₀ ), the film is a grounded resistor or capacitor having a value of Z ₀ / 2x, and the space behind the film is grounded Another Z ₀ resistor. If the free space behind is replaced with a dielectric such as food, the second Z ₀ must be replaced with the dielectric impedance (Z _d ). For vertically entering radiation, the ratio of Z _d to Z ₀ is 1 / ε _r ^1/2, so by a simple circuit representation, the transmission coefficient for a dielectric with a capacitive film coating is given by:

T = 2 / (1 + 2x + ε _r ^1/2 ) (7)

In resistive films, x is a real number, so T decreases monotonically with x. If the dielectric is lossy, ε _r has a negative imaginary component. Thus, for capacitive films (where x is an imaginary number), when | x | is first increased, the x term begins to cancel the imaginary part of ε _r and T actually increases. Eventually, x dominates ε _r and T falls, but for some time the capacitive film improves the impedance matching of the lossy food and consequently increases the energy input to the food. Once T is known, the fraction of energy transmitted into the load of the dielectric food can be calculated as the real part of ε _r ^1/2 TT ^* . Here, T ^* is a conjugate complex number of T.

When the impedance matching films of the present invention are separated by a distance L, the absorption of microwave energy by food is greatly increased. Using the transmission line impedance equation to transfer the impedance of the dielectric through the free space to the film by a distance L, Z _d can be replaced with Z _d as a function of L, as given below. .

In the above equation (8), k ₀ is the wave number in free space equal to 2πf (ε ₀ μ ₀ ) ^1/2 , and μ ₀ is equal to 4π × 10 ⁻⁷ henry / meter. By replacing Z _d / Z ₀ from Eq. (8) with Eq. (7) for 1 / ε ^1/2 , the capacitive film is fairly well shielded at a film-dielectric spacing of an integral multiple of half wavelengths. I knew it was possible. Almost complete when spacing is about 1 cm (+ integer multiple of half wavelength) and x is about 1.0i (or the product of dielectric constant and thickness for vertical radiation at 2.45 GHz is about 0.04 meters) Absorption is achieved with an infinite load.

Using the circuit model described above, the effective load of the film and load (eg water) is a parallel combination of film and load transmitted to the film. Accordingly, the reciprocal of the effective load is the sum of the film impedance and the reciprocal of the transmitted load impedance. When equation (8) is used to transmit impedance (Z) as a function of L, the impedance normalized to Z ₀ (and its reciprocal) is the real axis | Z | / Z ₀ and Z ₀ / Draw a circle on the complex impedance plane cut at | Z |.

At some point along the curve, ie at a certain distance L, the reciprocal of the normalized impedance is the addition of the positive imaginary number Ni at 1.0. When a film is selected where x is i / N, the reciprocal of x is -Ni, the total impedance is Z ₀ , a perfect impedance match where no energy is reflected. Since the capacitive film of the present invention does not absorb, all energy eventually becomes heat during loading. For this reason, it is very effective for heating the inside of highly dielectric foods such as pot pie and fruit pie.

  The value of x for total absorption at the appropriate interval is expressed below as a function of the dielectric constant of the food.

As a result, for foods with high dielectric constants, the best film capacity for impedance matching depends somewhat on the fourth root of | ε _r | −1. Thus, the volume is not extremely sensitive to ε _r and a single film works effectively over a wide range of food loads.

Example 1
The above model was experimentally tested in a microwave oven using a grounded circular waveguide as a container for water loading. The waveguide was 8.5 cm in diameter and the water level was 3.5 cm. Capacitive films made according to the present invention (x = 1.4i and x = 0.8i) were laminated to paperboard and cut into circles with a diameter slightly less than 8.5 cm. Circular capacitive films were placed in the waveguide at various levels above the water, and the temperature increase after 2 minutes was recorded in a 650 watt microwave oven. This temperature increase was compared with the temperature increase when a bare plate was placed at the same position. The results are shown in Table 1 and Table 2 below.

  It can be seen that the bare plate temperature change decreases slightly with the interval. However, when comparing the capacitive film of the present invention to a bare plate, ie, a bare plate without a capacitive film, in each example, the heat absorption of water is increased by more than twice at shorter intervals. At intermediate intervals, as expected, no apparent effect of the capacitive film was seen.

Avery Dennison Corporation produces aluminum flakes having an aspect ratio of at least 1000 that provides the x value required for the present invention in practical thickness films. In particular, preferred aluminum flakes useful for the present invention are produced by the Decorative Films Division of Avery Dennison Corporation, METALURE ^™ L-57083, L-55350, L-56903, L-57097, L-57103 and It has the product name L-57102.

These particular flakes are produced by vacuum depositing a metal layer on a thin soluble polymer coating applied to a smooth support. Preferably, a biaxially stretched polyester type film such as MYLAR ^™ (product of DuPont) is used as the support. The metal layer formed on the support is peeled off by dissolving the soluble coating. The preferred deposition thickness of aluminum metal gives an optical density of 1-4 before stripping. This provides flakes having the desired shape and dimensions. If the deposited metal film is too thin, the flakes will not have sufficient strength to prevent rolling up during peeling. On the other hand, if the deposited metal film is too thick, the surface of the film tends to give flakes a rough surface. Following stripping, the metal layer is mechanically mixed to provide the desired flake particle size while substantially preventing flake breakage.

  The flakes typically have a larger dimension or length average of 8 to 75 μm, with a very small amount of fine flakes with the larger dimension being less than 5 μm. Preferably, the flake width falls within the range of 5 to 35 μm. The fine particles help to keep the flake surface away. The following are the average length and width dimensions of the flakes as measured by a Dapple Image Analyzer.

  Although L-57103 and L-57102 flakes are microwave responsive, these flakes are difficult to coat and are therefore not the most preferred flake materials for impedance matching. However, these flakes are preferred flake materials to provide microwave shielding, discussed in more detail below.

  The difference between the preferred Avery type flake material and the commercially available flake material is readily apparent when viewed under a microscope. Other commercially available metal flake materials have a sufficient aspect to provide the high dielectric constant required to properly impedance match the microwave energy entering the food in a thin film to uniformly heat the center. Has no ratio and flatness. To show this difference, the commercial flake material was expanded and visually compared to the preferred Avery flake material, showing a clear difference therebetween.

  FIGS. 13-15 show STAPA-C VIII type aluminum flakes produced by Obron Corp. and FIGS. 16-18 show ALCAN 5225 type aluminum flake materials produced by Alcan. As is apparent from these photographs taken at x3000 and x8000, these materials have a smaller surface area than the Avery flakes shown in FIGS. This results in an aspect ratio of only 75-80 for ALCAN 5225 flakes and about 200 for STAPA-C VIII flakes. Avery flakes have a large surface area and are also very thin to provide Avery flakes with a higher aspect ratio and eventually a higher dielectric constant than other aluminum flake materials when immersed in a binder. Furthermore, Avery flakes have smooth parameter edges that are considerably rounder than the rough edges exhibited by conventional flake materials and contain fewer flake debris.

  The aluminum flake material produced by Avery is important for the operation of the impedance matching film of the present invention, mainly due to the extremely high dielectric constant provided by these flakes. A performance comparison between Avery aluminum flakes and aluminum flake materials produced by other manufacturers clearly shows the distinct advantages of Avery-type flake materials when the total aluminum content is the same. Tests were performed to compare the x values mathematically described above for a number of conventional flake materials and an Avery flake sample.

Example 2
7.78 g Dow Corning 1-2577 conformal coating (5.6 g silicone resin solids in toluene) was added to 30.3 g toluene and 1.4 g Hercules ethylcellulose (T-300 dissolved in 29.7 g toluene). Grade). A mixture of Alcan 5225 (particle size 12-13 μm, 65% solid aluminum flake paste in isopropyl alcohol) and 60 g of ethyl acetate was stirred until a uniform dispersion was obtained. Added to the mixture. The resulting formulation had a total solids content of 10% and an aluminum flake to binder ratio of 50/50. A sheet of polyester film (Melinex 813/92 from ICI) was coated with the formulation using a series of Bird film applicators.

  A similar formulation is pre-mixed with 11 g of STAPA-C VIII (particle size 11 μm, 60% solid aluminum flake paste in isopropyl alcohol) with 12.5 g of ethyl acetate until the flakes are evenly dispersed. Manufactured by. To this, 7.8 g of Dow Corning 1-2577 conformal coating (5.6 g silicone resin solid in toluene), 30.3 g toluene, 1.4 g Hercules ethylcellulose (T-dissolved in 29.7 g toluene) 300 grade) was added. The resulting formulation was 10% solids and the aluminum flake to total binder ratio was 50/50. This formulation was also applied to the polyester sheet film as described above.

A similar mixture was formed using the preferred Avery flake material L-56903. As described in detail in Example 7 below, an aluminum flake to total binder ratio of 50/50 was formed. The x value of 2.45 GHz for normally incident radiation (Z ₀ = 377 ohms) is, for example, equations (3) and (4) and a network analyzer on a sample mounted sideways in an S-band waveguide Calculated using transmission and reflection measurements. The results for these three sheet materials are shown in FIG. 19 as a function of aluminum coating amount.

FIG. 19 clearly shows that the use of these conventional aluminum flake materials is less practical to achieve the impedance matching capability of the thin film of the present invention than flake materials having Avery flake properties. In particular, 20-40 pounds per 3000 square feet (approximately 2.79 × 10 ² m ² ) to reach the desired x value of 0.7i-2.0i, more preferably 1.0i-1.8i. Conventional flakes (about 9.07-18.14 kg) are required. Thus, an excessive amount of flake material cannot easily form a thin film. Furthermore, even at this extremely high level, there is no indication that such a large amount of flake material can actually perform the impedance matching function of the present invention.

  In addition, further tests were performed to compare gravure printing possibilities for the preferred flake materials in silicon and acrylic binders and the conventional flake materials in silicon binders.

Example 3
The coating was made by mixing 5000 g of toluene and 4000 g of aluminum flakes (Metal L-56903, 10% solids in ethyl acetate). To this was added a mixture of 556 g of Dow Corning 1-2577 (73% solids in toluene), which is a silicone resin, and 444 g of toluene. The resulting formulation was 8% solids and the ratio of aluminum flakes to binder solids was 1: 1. The viscosity of the formulation was 22 seconds with a # 2 Zahn cup. This formulation was applied to a PET film (grade 813/92 from ICI) on a web-feed gravure press at 113 feet (about 34.5 m) / min using a 100 line cylinder with etched square cells. .

Example 4
The coating was made by mixing 3360 g aluminum flakes (Metal L-56903, 10% solids in ethyl acetate) and 1920 g n-propyl acetate. To this mixture was added 108 g of Johnkrill SCX-611 (acrylic resin from SC Johnson & Sons) in 252 g of n-propyl acetate and 36 g of ethylcellulose in 324 g of n-propyl acetate (grade N- from Hercules). 300) was added. This mixture was diluted to 6% total solids by adding an additional 2000 g of n-propyl acetate. The viscosity of the resulting mixture was 24 seconds with a # 2 Zahn cup. The resulting mixture was applied to a PET film using a gravure press as described above in Example 3 at a line speed of 125 feet (about 38.1 m) / min.

Example 5
In addition, a coating using a conventional aluminum flake material is prepared by first mixing 3200 g of STAPA-C VIII (65% solid paste in isopropyl alcohol) with 2300 g of ethyl acetate and 1000 g of isopropyl acetate until a uniform dispersion is obtained. It was done. To this dispersion was added a mixture of 1250 g Dow Corning 1-2577 (72% solids in toluene) and 2250 g toluene. The combined formulation was 30% solids and had a viscosity of 17 seconds in a # 2 Zahn cup. The resulting mixture was applied to a PET film using a gravure press at a line speed of 75 to 85 feet (about 22.9 to 25.9 m) / min as described above in Example 3. For the formulations of Examples 3-5, the resulting coating weights and x values for vertical radiation at 2.45 GHz are given in Table 4 below.

  The effect of the flake size of the preferred aluminum flake material having the characteristics of flakes produced by Avery on the x value is also important in achieving the desired impedance matching characteristics. Similar to formulations using STAPA-C VIII flakes from Oblon, a number of coating formulations were produced using each of the flakes obtained from Avery.

Example 6
The coating formulation was prepared by mixing 56 g of solid 10% aluminum flake slurry (Metal L-55350) in ethyl acetate and 32 g of n-propyl acetate. To this, 1.8 g Joncrill SCX-611 (acrylic resin from SC Johnson & Sons) in 4.2 g n-propyl acetate and 0.6 g ethylcellulose in 5.4 g n-propyl acetate ( Grade N-300) from Hercules. This 8% solids formulation with an aluminum flake to binder ratio of 70/30 was applied to PET film using a Bird bar applicator, resulting in the coating weights shown in Table 5 below.

  The general procedure was repeated for L-57083, L-56903, L-57103, L-57102, and STAPA-C VIII flake materials. The results of this comparison are given in Table 5 below and are shown graphically in FIG. The results of this comparison show that within the preferred Avery flake flake size range, all of which are superior to conventional flakes, the 17 μm flake size consistently provides the best capacitive x value for impedance matching. It shows that. The results in Table 5 also show that the very effective dielectric constant achievable with the present invention exceeds 18000 compared to only 1000 for the prior art material.

  It is also important to use a suitable flake to binder ratio to achieve the desired x value with the preferred flakes. The following tests were performed to show the effect of the proportion of aluminum flake material in the binder on the x value. It is believed that as the amount of binder in the capacitive film increases, the spacing between flakes increases as well. Generally, the flakes can constitute about 30-80 weight percent of the film to achieve the advantageous effects of the present invention. Preferably, the flakes are present from about 30 to 70 weight percent.

Example 7
A masterbatch of aluminum flake coating was prepared using silicon resin as the first binder, thickener and ethyl cellulose as the second binder. Masterbatch includes 4.44 g Dow Corning 1-2577 conformal coating (3.2 g silicone resin solid in toluene) and 2.8 g Hercules ethylcellulose (T-300 grade pre-dissolved in 59.2 g toluene). And was included. To this mixture, 14 g of aluminum flake solid (L-56903 in 10% solids ethyl acetate) was added. Therefore, the aluminum flake to binder ratio was 70/30.

(1) 70/30 aluminum flake to binder coating:
51.5 g of the masterbatch (containing 5 g of bound solids) was diluted to 100 g with toluene. The wet film of this 5% solid formulation was applied to a polyester film sheet (MELINEX 813/92) using a bird film applicator. By using an applicator designed to apply 0.0005, 0.001 and 0.002 inch (about 0.0127, 0.0254 and 0.0508 mm) wet films, respectively, 3000 square feet (about 0.4, 0.8 and 1.5 pounds (about 1.81 × 10 ⁻¹ , 3.63 × 10 ⁻¹ and 6.80 × 10 ⁻¹ kg) per 2.79 × 10 ² m ² ) A dry coating containing aluminum flake solids could be obtained.

(2) 50/50 aluminum flake vs. binder coating:
To the above master batch 36.8 g (including aluminum flakes 2.5 g, silicon resin 0.57 g, and ethyl cellulose solid 0.50 g), Dow Corning 1-2577 silicon resin solution 1.7 g (solid 1.23 g), Hercules 0.2 g ethyl cellulose (T-300 grade dissolved in 4.3 g toluene) and 52 g toluene are added to provide a 5% total solids formulation containing 50% aluminum flakes and 50% total binder. It was done. This formulation is applied to the film using the above techniques, 3,000 square feet (about 2.79 × 10 ² m ²⁾ per 0.7,1.2 and 2.0 pounds (about 3.18 × 10 ^{- 1} , 5.44 × 10 ⁻¹ and 9.07 × 10 ⁻¹ kg) of aluminum flake solids were obtained.

(3) 30/70 aluminum flake to binder coating:
To the masterbatch 22.1 g (containing 1.5 g of aluminum flakes, 0.34 g of silicon resin, and 0.30 g of ethyl cellulose solid), 3.4 g of Dow Corning 1-2577 silicon resin solution (solid 2.46 g), Hercules Formulation of 5% total solids containing 0.4 g ethylcellulose (T-300 grade dissolved in 8.5 g toluene) and 65.6 g toluene, including 30% aluminum flakes and 70% total binder. Was manufactured. This formulation is applied to the film using the above techniques, 3,000 square feet (about 2.79 × 10 ² m ²⁾ per 0.6,1.0 and 1.3 pounds (about 2.72 × 10 ^{- 1} , 4.54 × 10 ⁻¹ and 5.90 × 10 ⁻¹ kg) of aluminum flake solids were obtained.

The x value of each coating was calculated from measurements made using an S-band waveguide and a Hewlett Packard network analyzer (model 8753A) as described above. The results are shown in Table 6 below and shown graphically in FIG. As is readily apparent from these results, the x value per pound of aluminum (about 4.54 × 10 ⁻¹ kg) improves as the flake ratio increases.

  Many more tests were performed with actual food samples to demonstrate the improved heating provided by the impedance matching member 22 of the present invention. In the following example, a food carton similar to the carton 10 of FIG. 1 was used.

Example 8
An elliptical impedance matching member 22 was placed 5/8 inch (about 15.88 mm) above the Tyson 18 oz (about 504 g) chicken pot pie. Control cartons with widths of 8 and 7/8 inches (about 203 mm), depths of 6 and 1/8 inches (about 156 mm), and heights of 1 and 1/2 inches (about 38.1 mm) were used. The control carton does not include an impedance matching member. A modified carton 10 similar to the carton shown in FIG. 1 was 1 and 7/8 inches in height (about 47.6 mm). The elliptical impedance matching member 22 had a length of 3 and 1/2 inch (about 88.9 mm), a width of 2 and 7/8 inch (about 73.0 mm), and x = 1.01i. Each run involves heating the pot pie for 5 minutes, rotating the pot pie 90 °, and then heating the pot pie for an additional 5 minutes.

  Four cooking runs are carried out, and the pot pie has no box (# 1), inside the control box (# 2), inside the box covered with the impedance matching member 22 on the entire inner surface (# 3), and in FIG. Cooked in a box (# 4) containing an oval member 22 positioned on the top panel as shown. The temperature probe was placed in the pot pie at the position shown in FIG. The results of these runs are shown in Table 7 below.

Example 9
Control carton (# 5) having no impedance matching member, rectangular impedance matching member 3, 1/2 inch (about 88.9 mm) × 3 inch (about 76.2 mm) (# 6), and the elliptical shape described above Another series of tests was performed to compare the impedance matching member 22 (here x = 0.8i). The pot pie is cooked as described above in Example 8 in each carton and the results of these runs are shown in Table 8 below.

Example 10
Test (# 8) was also performed using the same elliptical conventional aluminum foil pieces provided above for the impedance matching member 22 used in Examples 8 and 9 above. The aluminum foil ellipse was lifted to 3/8 inch (about 9.53 mm) above the Tyson 18 oz pot pie.

Example 11
The test (# 9) was performed using an impedance matching member 22 having a thickness (x = 1.3i + 0.8i) twice that of the impedance matching member and the same elliptical shape as described above.

Example 12
Test (# 10) was performed using an enlarged elliptical impedance matching member 22 (here x = 1.3) having dimensions of 4 inches × 4 and ½ inch (about 114 mm). Other conditions were the same as above.

Example 13
Also, the distance of the impedance matching member 22 having an elliptical dimension of 3 inches (about 19.1 mm) × 3 and 1/2 inch (about 88.9 mm) was adjusted to determine the central pie heating (# 11 ). In particular, the member was placed inside the top surface of the carton 1/2 inch above the surface of the pie. The results of Examples 10-13 are given in Table 9 below.

Example 14
The dimensions of the carton 10 and the member 22 were adjusted to optimize the degree of heating at the center of the pot pie (# 12). For example, an open end carton or sleeve having a length of 9 inches (about 229 mm), a width of 6 inches (about 152 mm) and a height of 2 and 1/4 inches (about 57.2 mm) heats a Tyson 18oz chicken pot pie. Used for. The pot pie was placed on three layers of corrugated paper and the distance between the pie and the impedance matching member was 5/8 inch (about 15.88 mm). A larger elliptical impedance matching member of 4 and 1/2 inch (about 114 mm) × 4 inch (about 102 mm) with x = 1.1i was used.

Example 15
A test (# 13) similar to Example 8 was performed using the same cooking sleeve. However, the dimensions of the elliptical impedance matching member were reduced to 2 inches (about 50.8 mm) × 1 and 3/4 inches (about 44.5 mm), where x = 1.1i.

Example 16
Two further tests (# 14 and # 15) similar to Examples 8 and 9 were performed using the same cooking sleeve. However, the dimensions of the elliptical impedance matching member were 2 and 1/2 inches (about 63.5 mm) × 2 inches (about 50.8 mm) with x = 1.1i.

Example 17
Finally, a control test (# 16) was performed with the same pot pie used in Examples 14-16. However, the pot pie was cooked without a carton. The results of Examples 14-17 are given in Table 10 below.

  The carton has also been tested to determine the optimum size of a rectangular or square impedance matching member that raises the temperature of the pot pie as well as the advantageous heating provided by the elliptical design. The series of tests uses a carton similar to that used in Examples 14-17 above with a carton depth of 1 and 5/8 inch (approximately 41.3 mm), and a rectangular member 2 instead of an elliptical impedance matching member. And 1/2 inch (about 63.5 mm) × 2 inches (about 50.8 mm). Table 11 gives the results of three different tests performed on the rectangular member (# 17, # 18, # 19, # 20). The control test was performed without a carton (# 21).

  As can be seen in each of the above results, a substantial increase in the center temperature of the pot pie was achieved using the impedance matching member of the present invention.

  The impedance matching member of the present invention is also useful for changing the relative cooking speed and temperature of two different food products. Such a result would be very effective in a finished microwave dinner containing a variety of different foods, each requiring different heating characteristics. For example, the meat portion of the finished dinner will require a higher heating temperature than the vegetable portion. However, in order to provide additional convenience to consumers, these food products are generally provided in the same packaging tray. By using the impedance matching member of the present invention in one part of the tray and not in the other part, a significant difference in temperature can be brought about.

Example 18
Two beakers with water were placed in a 600 watt microwave oven at the same time. One beaker was placed on the left side of the microwave and the other was placed on the right side. The average power absorption from room temperature to boiling was calculated for each beaker. For all possible combinations, i.e. when not using impedance matching members, the left side is impedance matched and the right side is not matched, the left side is not matched, the right side is impedance matched, and both sides are impedance matched, Data was taken. The experiment was performed for both a 100 mL water load and a 400 mL water load. The results are shown in Table 12 below.

  The impedance matched portion of the range contents was heated faster than the unmatched portion. However, if the entire contents are impedance matched, the entire range output is not increased. Partial impedance matching generally redistributes heating within the range.

  In addition to the uniform impedance matching member used to impedance match the radiation into the hard to heat the area of food, the impedance matching member of the present invention functions in a microwave oven as well as a convex glass lens. Thus, it can be formed non-uniformly. FIG. 23 shows an example of a modified impedance matching member 22 'in the package 10 that is shaped like a convex optical lens. Such a shape is useful for further directing microwave radiation to a desired area of the package 10.

  As described above, the transmission coefficient is a complex number. Therefore, there will be a phase shift due to the film represented by equation (10).

Φ = -tan ^-1 x (10)

  When the impedance matching member of the present invention is printed so that the center is thicker than the edge, the phase shift is reduced as it approaches the periphery of the member. As a result, the microwave radiation is focused in the same way as the light passing through the convex optical lens.

  In particular, as in the case of optical lenses, focus conditions arise due to a phase shift at the center equal to the extra shift due to the greater stroke depth at the edge. That is, it is shown by Formula (11).

tan ⁻¹ x = 2π [(h ² + L ² ) ^1/2 −1] / λ (11)

  Here, h is half the height of the lens, L is the focal length, and λ is the wavelength of radiation. In order to realize the best lens shape formed according to the present invention, the x value of the lens as a function of y (distance from the lens center), the following equation is applied:

  In addition to the above advantages of impedance matching, the film can also act as a shield if the x value of the film is sufficiently high. In particular, when the x value is higher than 10i, for example, the film functions as a shield, and the amount of microwave energy that reaches food disposed below the film can be reduced. For vertically entering radiation, the ratio of the electric field amplitude entering a dielectric food having a capacitive film shield on the surface to the electric field entering a food without such a shield is expressed by the following equation (13).

Here, ε is an effective dielectric constant. As evidenced by this relationship, the level of capacitive film depends on the dielectric constant. For a typical food product with a dielectric constant of 50, the capacitive x value is at least 10i. Table 5 gives examples of flake materials and coating amounts that can provide shielding. In particular, L-57103 flakes with an average length of 25 μm and 1.0 to 1.7 pounds (about 4.54 × 10 ^{−1 to} 7.72 ×) per 3000 square feet (about 2.79 × 10 ² m ² ). The coating amount is 10 ⁻¹ kg).

Example 19
Tests were performed to demonstrate the usefulness of high x value capacitive films for shielding food in microwave ovens. In particular, two paper cups containing 120 g of water were each placed in a 700 watt Litton microwave oven. Initially, each cup of water that did not incorporate flake material in the cup was heated in a 700 watt LITTON ^™ microwave until one reached about 200 ° F. The temperature in each cup was monitored with two Luxtron probes suspended in a reproducible position in water. The average heat loss expressed in watts was calculated for each cup of water from the average temperature rise and heating time. The aluminum foil patch was then glued to the bottom and sides of one cup shown as cup B. Again, the average power loss was calculated. This procedure was performed two more times using capacitive films with x values of 1.5i and 20i, respectively, instead of aluminum foil patches. The results are shown in Table 13 below.

  As can be seen from these results, the 1.5i film has little effect on power loss when placed on the container surface. However, the aluminum foil provides significant shielding, as shown by the reduced power loss of test 2 cup B. As shown by Test 4, the 20i film also provides shielding, and by using a capacitive film made according to the present invention, it is also demonstrated that the amount of shielding can be controlled by adjusting the x value of the film.

  The above description is considered as illustrative only of the principles of the invention. Further, since various modifications and changes can be easily made by those skilled in the art, the present invention is not limited to the exact configuration shown and described. Accordingly, all suitable modifications and equivalents are within the scope of the invention.

It is the food package containing the microwave impedance matching element of this invention. FIG. 2 is an exploded cross-sectional view taken along line 2-2 of the package of FIG. FIG. 4 is an exploded cross-sectional view of a second embodiment of the package of FIG. 1. FIG. 6 is an exploded cross-sectional view of a third embodiment of the package of FIG. 1. FIG. 6 is an exploded cross-sectional view of a fourth embodiment of the package of FIG. FIG. 7 is an exploded cross-sectional view of a fifth embodiment of the package of FIG. 1. FIG. 10 is an exploded cross-sectional view of a sixth embodiment of the package of FIG. 1. FIG. 10 is an exploded cross-sectional view of a seventh embodiment of the package of FIG. 1. FIG. 10 is an exploded cross-sectional view of an eighth embodiment of the package of FIG. 1. It is sectional drawing of another Example of the food package containing the microwave impedance matching element of this invention. 2 is an enhanced microscopic image of the aluminum flakes of the present invention. 2 is an enhanced microscopic image of the aluminum flakes of the present invention. 1 is an enhanced microscopic image of a prior art aluminum flake. 1 is an enhanced microscopic image of a prior art aluminum flake. 1 is an enhanced microscopic image of a prior art aluminum flake. 1 is an enhanced microscopic image of a prior art aluminum flake. 1 is an enhanced microscopic image of a prior art aluminum flake. 1 is an enhanced microscopic image of a prior art aluminum flake. It is a graphical comparison between a capacitive film containing the aluminum flakes of the present invention and a film containing other less effective aluminum flakes. It is a graphical comparison between a capacitive film containing the aluminum flakes of the present invention and a film containing other less effective aluminum flakes. 3 is a graphical comparison of capacitive films containing aluminum flakes of the present invention at different binder to flake ratios. Figure 5 shows the temperature probe position within the sample food used in the example above. It is a decomposition | disassembly cross-sectional side view of 2nd Example of the microwave impedance matching element of this invention.

Explanation of symbols

10: food package 12: food 14: food receiving space 16: top panel 18: side panel 20: bottom panel 22: impedance matching member 24: substrate 26: adhesive layer

Claims (6)

A composite material for shielding food from the microwave energy placed closest,
(A) a substantially microwave energy permeable substrate;
(B) Shielding means provided on at least a portion of the substrate to reduce the amount of microwave energy reaching the food disposed closest, where the composite material is disposed closest Shielding means comprising a continuous film of generally planar flakes embedded in a dielectric binder in an amount sufficient to reduce microwave energy reaching the food;
A composite material for shielding microwave energy. The composite material for shielding microwave energy according to claim 1, wherein the capacitive x value of the composite material is larger than 10i. The composite for microwave energy shielding according to claim 2, wherein the flakes comprise aluminum having a longest average dimension in the range of about 8 to 75 micrometers. The flakes are within the range of about 1.0 to 1.7 pounds (about 4.54 × 10 ^{−1 to} 7.72 × 10 ⁻¹ kg) per 3000 square feet (about 2.79 × 10 ² m ² ). The composite material for shielding microwave energy according to claim 3, wherein the composite material is present in the binder. The composite material for microwave energy shielding according to claim 4, wherein the shielding means has an effective dielectric constant of at least about 100,000. 6. The composite for microwave energy shielding according to claim 5, wherein the flakes have an aspect ratio of at least about 1000.